1. Field of the invention
The present invention relates to a process for dewaxing waxy heavy distillate and residual petroleum oil stocks. More particularly, the invention relates to a solvent dewaxing process wherein the waxy charge stock is prediluted with solvent in an amount equivalent to about 1 to 2.5 volumes of charge, wherein the mixture is cooled to about the depressed cloud point and wherein the mixture is subsequently diluted with solvent in increments equivalent to about 0.3-1.5 volumes of charge each to a total of about 3.5 to 4 volumes solvent per volume oil under conditions of plug flow radial mixing and cooling at a rate of about 1.degree.-8.degree. F./min (0.56.degree.-4.4.degree. C. min), wherein said incremental additions of solvent are at a temperature in the range of about 60.degree.-90.degree. F. (15.degree. to 32.degree. C.) and wherein the diluted oil is cooled at 1.degree.-8.degree. F./min (0.56.degree.-4.4.degree. C./min) to a desired separation temperature for crystalizing wax therefrom.
2. Description of the Prior Art
It is known in the prior art to dewax waxy petroleum oil stocks by cooling oil-solvent solutions at uniformly slow rates, of e.g. 1.degree.-8.degree. F./minute (0.56.degree.-4.4.degree. C./min) under controlled conditions for crystalization of wax from said solutions. Commercially, such oil-solvent solutions are cooled by several methods such as indirect heat exchange in scraped surface exchangers; dilution chilling wherein waxy oil stock is contacted in a multi-stage tower with chilled solvent under conditions of high levels of agitation (U.S. Pat. No. 3,773,650); and direct chilling, wherein a low boiling solvent, e.g. propylene, mixed with waxy oil stock is vaporized under conditions of reduced pressure.
In such commercial processes, the waxy oil charge, or solutions of waxy oil charge and solvent, are heated to a temperature at which all the wax present is dissolved. The heated charge is then passed into a cooling zone wherein cooling is undertaken at a uniform slow rate in the range of about 1.degree.-8.degree. F./minute (0.56.degree.-4.4.degree. C./min) until a temperature is reached at which a substantial portion of the wax is crystalized, and at which dewaxed oil product has a selected pour point temperature. Upon achieving the desired dewaxing temperature, the mixture of wax crystals, oil and solvent is subjected to solid-liquid separation for recovery of a wax free oil-solvent solution, and a solid wax containing a minor proportion of oil (slack-wax). The separated oil-solvent solution is subjected to fractional distillation for recovery of solvent fraction and product dewaxed oil fraction. The slack wax may be recovered as is, or may be subjected to additional processing, such as repulp filtration for removal of additional oil therefrom.
Solid-liquid separation techniques which may be employed for separation of wax crystals from the oil-solvent solutions include known solid-liquid separation processes, such as gravity settling, centrifugation, and filtration. Most commonly, in commercial processes, filtration in a rotary vacuum filter, followed by solvent wash of the wax cake, is employed.
Dewaxing solvents which may be used in solvent dewaxing processes include known dewaxing solvents. Commonly used solvents include aliphatic ketones of 3-6 carbon atoms, C.sub.2 -C.sub.4 range hydrocarbons, C.sub.6 -C.sub.7 aromatic hydrocarbons, halogenated C.sub.1 -C.sub.4 hydrocarbons and mixtures of such solvents. Solvent dilution of waxy oil stocks maintains fluidity of the oil for facilitating easy handling, obtaining optimum wax-oil separation and obtaining optimum dewaxed oil yields. The extent of solvent dilution depends upon the particular oil stocks and solvents used, the approach to filtration temperature in the cooling zone, and the desired final ratio of solvent to oil in the separation zone.
For processes employing indirect cooling in scraped surface exchangers, cooling and wax crystalization is accomplished under conditions of very little agitation at a rate in the range of about 1.degree.-8.degree. F. minute (0.56.degree.-4.4.degree. C./min). Under such conditions, without wall scrapers, wax tends to accumulate on the cold exchanger walls, interfering with heat transfer, and causing increased pressure drop. Thus, scrapers are employed to remove the accumulated wax. Dewaxing solvents are employed to maintain fluidity of the oil in the coolers and chillers, and may be added before the oil is cooled or in increments during cooling. Often the oil is given a final dilution with solvent at the separation temperature for reducing solution viscosity such that wax separation is more efficient. Commonly, solvent added to the oil in such processes is at the same temperature, or somewhat higher temperature than the oil. Cold solvent, added at substantially lower temperatures than the oil, shock chills the oil, resulting in formation of many small wax crystals which are difficult to separate. Under controlled conditions, elongated wax crystals of good size are formed which are easy to separate and which contain little occluded oil.
Dilution chilling processes employ incremental addition of cold solvent, eg. +20.degree. to -25.degree. F. (-6.7.degree. to -32.degree. C.), to the oil under conditions of high degrees of agitation, such that oil and solvent are completely mixed in less than one second. Under such conditions, wax precipitates in small, hard balls rather than elongated crystals. Such wax precipitates are easy to separate and retain very little oil.
Direct chilling processes employ a low boiling hydrocarbon, e.g. propylene, as dewaxing solvent and refrigerant. Waxy oil stock is diluted with sufficient low boiling hydrocarbon to provide the necessary cooling and provide the desired final dilution to facilitate separation of solid wax from the oil-solvent solution. The low boiling hydrocarbon is vaporized from the oil-low boiling hydrocarbon solution under conditions of reduced pressure, at a rate sufficient to cool the solution about 1.degree.-8.degree. F. per minute (0.56.degree.-4.4.degree. C./min). Such cooling is continued until the desired separation temperature and degree of wax crystalization are obtained. At the separation temperature, sufficient low boiling hydrocarbon remains in solution with the oil to provide the desired fluidity for good separation of wax. Agitation of the mixture being cooled is commonly provided for reduction of temperature and concentration gradients.
In these processes of the prior art, rotating mechanical equipment, either scrapers or high speed agitators, conditions, elongated wax crystals of good size are formed which are easy to separate and which contain little occluded oil.
Dilution chilling processes employ incremental addition of cold solvent, eg. +20.degree. to -25.degree. F. (-6.7.degree. to -32.degree. C.), to the oil under conditions of high degrees of agitation, such that oil and solvent are completely mixed in less than one second. Under such conditions, wax precipitates in small, hard balls rather than elongated crystals. Such wax precipitates are easy to separate and retain very little oil.
Direct chilling processes employ a low boiling hydrocarbon, e.g. propylene, as dewaxing solvent and refrigerant. Waxy oil stock is diluted with sufficient low boiling hydrocarbon to provide the necessary cooling and provide the desired final dilution to facilitate separation of solid wax from the oil-solvent solution. The low boiling hydrocarbon is vaporized from the oil-low boiling hydrocarbon solution under conditions of reduced pressure, at a rate sufficient to cool the solution about 1.degree.-8.degree. F. per minute (0.56.degree.-4.4.degree. C./min). Such cooling is continued until the desired separation temperature and degree of wax crystalization are obtained. At the separation temperature, sufficient low boiling hydrocarbon remains in solution with the oil to provide the desired fluidity for good separation of wax. Agitation of the mixing being cooled is commonly provided for reduction of temperature and concentration gradients.
In these processes of the prior art, rotating mechanical equipment, either scrapers or high speed agitators, are employed to facilitate good heat transfer from the oil. Such rotating mechanical equipment is expensive, difficult to maintain, and can contribute to breaking and deformation of wax crystals.